Nanowire-based chemical sensors have, in the last few years, excited great interest, leading to many developments being reported in the literature. In particular, sensors using silicon nanowires have been described for detecting the pH of a solution of proteins (Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M.; Science, 2001, 293, 1289-1292), or else gas molecules (Zhou, X. T.; Hu, J. Q.; Li, C. P.; Ma, D. D.; Lee, C. S.; Lee, S. T.; Chem. Phys. Lett., 2003, 369, 220-224) and other molecules of interest. The detection mechanism of these silicon-nanowire-based sensors is generally based on a change in the electrical conductivity of these nanowires in the presence of charged chemical molecules or biological molecules on their surface.
Detection of these target molecules is generally made possible only after an appropriate chemical functionalization of the surface of the silicon nanowires with “probe” molecules capable of interacting specifically with the target molecules to be detected. Most of the time, these sensors take the form of matrices in which the selective functionalization of the nanowires is vital.
Several methods aiming to localize the surface functionalization have been described in the literature, in particular: methods based on electrostatic attraction between the probe molecules (or receptor molecules) and the nanowire: photolithography of a hydrophobic film; microcontact printing; dip pen nanolithography; and electrochemical methods.
However, the electrostatic-attraction-based method does not allow a high selectivity for the surface modification to be obtained relative to the surrounding areas and may not be used to graft neutral or weakly charged receptor molecules.
The photolithographic and microcontact printing methods do not allow ultrafine, especially nanoscale, localization.
Furthermore, printing techniques also require a precise alignment of the prefabricated micro/nano structures. Dip pen nanolithography provides excellent flexibility in terms of localization of the selective surface functionalization and in terms of the molecules that can be grafted. However, this technique requires the use of expensive equipment, and it is not suitable for surface functionalization on a large scale. Finally, electrochemical methods are limited by the choice of functional groups, which must have redox properties.
More recently, a method for selectively functionalizing silicon nanowires using the Joule effect has been reported. More precisely, this method is based on the very localized nanoscale Joule heating that occurs when an electric field is applied across the terminals of a silicon nanowire (Inkyu Park; Zhiyong Li; Albert P. Pisano and R. Stanley Williams; Nano Leu., 2007, 7 (10), pp 3106-3111). This very localized heating can be used to selectively remove a protective polymer film that covers a preselected region of a silicon nanowire. After appropriate subsequent processing, the surface thus exposed can then be functionalized by chemical molecules, whereas the other neighbouring regions of the nanowire remain protected by the chemically inert polymer film.
However, this surface functionalization method requires a prior step of coating the silicon nanowires with a polymer film and then, optionally, a step of removing the protective polymer film once the surface functionalization has been carried out.